In the field of optical communication, there is an intensive use of Dense Wavelength Division Multiplexing (DWDM) in which plurality of information channels corresponding to plurality of different wavelengths are inserted, in parallel, into the same optical fiber. In communication systems, there is a need to transmit information from one channel having a certain wavelength to another channel with another wavelength. Accordingly, in such a situation where these channels operate in different wavelengths, there is a need for wavelength converters to allow the transmission of the information from one information channel to another information channel and without the need of Optical-Electrical-Optical (O-E-O) converters.
Wavelength converters may be used also for switching purposes when the wavelength change results with a different port from which the radiation is emitted by wavelength sensitive demultiplexers (WDM or DWDM).
FIG. 1 is a schematic illustration of a prior art Mach Zhender Interferometer (MZI) wavelength converter 400. Wavelength converter 400 is designed to convert information generating pulses 402 of wavelength λ1, at terminal 404, into converted information pulses 406 of wavelength λ2, at terminal 408. Continuous Wave (CW) radiation 410 having wavelength of λ2, is inserted at terminal 412 and is split by coupler 414 into CW radiation propagating in branches of radiation guides 416 and 418. The radiation in branched 416 and 418 passes through Solid-state Optical Amplifiers (SOA) 422 and 420, respectively, serving as Non Linear Elements (NLE). SOA's 420 and 422 are adjusted to produce relative phase shifts between the CW radiation in guides 416 and 418 for causing the radiation from guides 416 and 418 to be combined destructively in coupler 424. Accordingly, when no signal 402 is present in terminal 404, there is no output signal 406 at port 408.
When signal 402 having wavelength λ1, is received by terminal 404 it is coupled, by coupler 426, into guide 418 and passes through SOA 420. For the time duration in which signal 404 passes through SOA 420, it causes a phase change of π radians to the CW radiation propagating, in the opposite direction in SOA 420. In this case the CW radiation from guides 416 and 418 is combined constructively, by coupler 424, to produce pulse 406 at output 408 having wavelength λ2 and time duration that is equal to the time duration of pulse 402. Accordingly, converter 400 converts pulses 402 of wavelength λ1, at port 404, into similar pulses 406, of wavelength λ2, at port 408.
Wavelength converter 400 may have the following disadvantages:
1. The device is phase sensitive and thus electric current injected to SOA's 420 and 422 should be controlled, separately, to each of them to maintain the desired phase relations and to compensate for phase changes resulted from environments changes and/or various drifts in the values of some parameters of device 400, such as gain drifts of SOA's 420 and 422.
2. In the absence of signals 402 at port 404 and in order to produce zero output signals 406 at port 408, there is a need to maintain independent gain and phase relations between amplifiers 422 and 420 at branches 416 and 418, respectively. However the gain and the phase shifts of amplifiers 422 and 420 are dependent parameters, resulting with a situation that may be difficult to control.
3. The design of the device requires two SOA's may reduce manufacturing yield, and may increase manufacturing cost and complexity.